The present invention relates generally to uninterruptible power supplies (“UPS”), and more particularly, to methods and apparatus for maximizing the battery runtime in a parallel UPS system.
Uninterruptible power supplies are designed to provide continuous power to a critical load in the event of a temporary failure of utility power. In some systems, a plurality of UPS modules are connected together in parallel to supply the critical load. Each UPS module generally includes a rectifier, a battery, and an inverter. In the event of an interruption of utility power, the critical load will be supplied by DC power from the batteries in the UPS modules. This DC power is converted to AC power by the inverters on the UPS modules before being passed to the critical bus. One disadvantage of a system using parallel UPS modules is that the load is distributed evenly across each of the batteries in the UPS modules. There is no guarantee, however, that the batteries will have the same energy delivery capabilities. For example, one battery may be relatively new and able to deliver twice as much runtime as another battery that is relatively old. So the UPS with the “old” battery may terminate operation due to an earlier End of Discharge (“EOD”) before the UPS with the new battery. In a system with multiple UPS modules, the module with the weakest battery may shut down first, followed by the next weakest, and so on. Each time a UPS module shuts down, the remaining modules experience an increased share of the load. This diminishes the runtime available for the remaining batteries and may overload the remaining UPS modules. The present application addresses these shortcomings and provides other advantages that will be apparent to those skilled in the art.